Microsoft has discontinued the Trackball Explorer, the best trackball ever, and the only pointing device I've ever been able to use without wrist pain. This is not good.
If this were a mouse, there'd be fifty competitors with nearly identical products. I'd just pick one at random and the problem would be solved. But the trackball market is tiny. There are only a handful of companies making trackballs, and each company seems to prefer a different style. The only trackball on the market today that even comes close to replicating the design of the Trackball Explorer is the Logitech Cordless Optical Trackman, but it has fewer buttons, the scroll wheel is in the wrong place, and the buttons it does have are small and more difficult to press.
I still have my original Trackball Explorer, and it still works just fine, but I wanted to get another one to use at work. I guess I'll have to settle for the Logitech at work, and I'll just have to hope my Trackball Explorer lasts long enough for someone else to release a similar product.